Semiconductor lasers are known that are configured to lase on a single longitudinal mode of the laser cavity, with the lasing mode and its wavelength being selected by a distributed Bragg reflector grating (DBR) having a narrow reflective peak. Such single longitudinal mode lasers have a side mode suppression ratio (SMSR) of at least 25 dB.
DBR lasers configured to operate at a fixed wavelength or to be tunable over a narrow wavelength range typically comprise a laser cavity defined between a broadband reflector (e.g. a broadband facet reflector) and a DBR section, and have within the laser cavity an optical gain section and typically a phase control section. The gain section is electrically driven to emit light by stimulated emission. The DBR section has a single, constant pitch grating configured to reflect light with a reflection spectrum having a single narrow reflective peak, with a full-width at half maximum that is equivalent to only a very small number of longitudinal cavity modes of the laser (e.g. less than five times the mode spacing, and preferably less than three times the mode spacing), which provides high mode selectivity and a correspondingly high SMSR. The phase control section has a tunable refractive index, with which to make fine adjustments to the optical path length of the optical cavity of the laser, in use, to control the relative alignment of the fine comb of laser cavity modes with respect to the narrow reflective peak of the DBR section's reflection spectrum (i.e. controlling the spectral operating position within the range of dominance of a particular cavity mode), in order to select the preferred lasing mode and to optimise its alignment with the reflective peak, in order to reduce the relative intensity of unwanted side modes. The wavelength tuning range of such lasers is limited by the maximum refractive index change that can be induced in the DBR section, which changes the effective pitch of the grating at the operating wavelength. The spectral width of a reflective peak narrows with increasing effective penetration length of the DBR section, and so for single longitudinal cavity mode operation, such DBR lasers have lengthy DBR sections.
Widely wavelength tunable DBR lasers are configured for tuning to lase across a wider wavelength range than is possible solely by tuning of the refractive index of a DBR having a single, constant pitch, and typically use two different, independently tunable DBR sections. Accordingly, such lasers typically have longer laser cavities than fixed wavelength or narrowly wavelength tunable DBR lasers, which consequently have greater sensitivity to their operating parameters, in particular with regard to stabilising the control of the lasing modes to avoid mode-hopping (jumping between lasing on different longitudinal cavity modes).
One such type of widely wavelength tunable DBR laser has a laser cavity formed between two DBR sections that produce reflection spectra of differently spaced combs of reflective peaks, which can be spectrally tuned relative to one another in a Vernier type of tuning arrangement, with lasing occurring at a wavelength common to both of the combs of reflective peaks (like the DBR laser with a pair of Vernier tunable phase-change gratings that is shown in FIG. 8 of U.S. Pat. No. 6,345,135). The DBR section at the output end of the laser typically has spectrally wider reflective peaks than the reflective peaks of the other DBR section.
A second such type of widely wavelength tunable DBR laser has a laser cavity formed between a first DBR section that produces a comb of narrow reflective peaks and a second DBR section that has a broad spectral band of reflectivities (e.g. a relatively uniform plateau) in the untuned state, but within which part of the reflection spectrum is wavelength tunable to overlap with another part, to produce a reinforced reflective peak that is several times broader (i.e. FWHM) than the narrow reflective peaks of the second DBR section, but narrower than the spectral separation of successive narrow peaks. This reinforced peak selects a narrow peak from the first comb-like reflection spectrum at which the laser will lase. Such a laser is described in U.S. Pat. No. 7,145,923.
As the spectral width of a reflective peak narrows, with increasing length of the corresponding DBR section, in both cases, the DBR section that is responsible for producing the narrow reflective peaks is longer than the DBR section producing the wider reflectiv peak(s).
A DBR section will reflect and transmit light in accordance with its reflection spectrum. However, a DBR section will also, undesirably, absorb some of the light incident into it. To minimise the absorption loss when light leaves the laser cavity, and to maximise the optical power output from the laser cavity, the long DBR section that has the narrow reflective peaks is typically the rear reflector of the laser cavity (i.e. at the opposite end of the laser cavity from the main beam output, at the front reflector).
In use, aside from heat generated in the sections of a DBR laser by the currents that drive the respective sections, heat is also generated in the DBR sections by optical absorption. Whilst heat is dissipated from the laser's optical waveguide, and is typically absorbed by a thermo-electric cooler (Peltier cooler) that is thermally coupled to the semiconductor chip, the rate of dissipation is not sufficient to avoid the creation of a thermal gradient in the DBR section, which affects the optical performance of the laser. In a DBR section, the local level of optical absorption induced heating corresponds to the local intensity of light, which decays approximately exponentially, throughout the DBR section, away from the optical gain section.
The effective refractive index of a semiconductor optical waveguide varies as a function of temperature. The inhomogeneous optical absorption in a DBR section induces corresponding inhomogeneous heating in the waveguide and consequently a corresponding variation in the effective pitch of the grating, in use. Further, thermal cross-talk from other sections, in particular from the gain section, can induce further inhomogeneous heating in a DBR section. Such optical absorption and thermal cross-talk may raise the temperature of the end of the DBR section that is closest to the gain section by 5° C. and the end of the DBR section that is furthest from the gain section by 1° C., for example, although the heating effect is greatest closest to the gain section, where it also increases most rapidly, towards the gain section. Thermal inhomogeneity induces inhomogeneity of the effective grating pitch in the DBR section, since the reflective wavelength of a grating may tune by about 0.1 nm/° C., and the effective chirp of the grating is greatest at the end of the DBR section closest to the gain section (e.g. approximately 3×10−4 nm/μm, along the part of the DBR section closest to the gain section). This effective inhomogeneity of the effective grating pitch broadens the width of a DBR section's reflective peak(s), reducing the optical performance of the laser under particular operating conditions (high gain section current and/or high DBR section tuning current). The effect of inhomogeneous thermally induced chirp is particularly significant in a DBR section that has a narrow reflective peak/peaks, and which is longer to provide a higher reflectivity.
The inhomogeneous thermally induced chirp in a DBR section that has a reflective spectrum of one or more narrow reflective peaks degrades the side mode suppression ratio (SMSR).